1. Field of the Invention
The present invention relates to a blasting apparatus suitable for carrying out a surface treatment of sintered products such as rare earth metal-based permanent magnets and ceramics.
2. Description of the Related Art
A blasting apparatus is conventionally used for a surface treatment of, for example, rare earth metal-based permanent magnets, i.e., a treatment for removing an oxide layer formed on the surface, a treatment for cleaning the surface or a shot peening for finishing a film formed on the surface.
There are various types of blasting apparatus. For example, in a tumbler-type blasting apparatus, work pieces are placed into a drum within the apparatus, and an injection nozzle is disposed so as to inject a blast material against the work pieces through an opening in the drum, while stirring the work pieces by rotating the drum (see Japanese Patent Application Laid-open No.11-347941).
A blasting apparatus as described above is capable of mass-treatment of work pieces and excellent in productivity. In such apparatus, however, the injection of the blast material against the work pieces can be conducted only through the opening in the drum, and hence, there is, of course, a limit in respect of the treating efficiency. When an attempt is made to stir the work pieces as homogenously as possible by prolonging the treating period of time or by increasing the rotational speed of the drum in order to enhance the treating efficiency, the collision of the workpieces against one another occur frequently and with a strong shock force. For this reason, a cracking and breaking is produced in many of the work pieces. The stirring of the work pieces must be carried out, so that they are not dropped out through the opening and hence, the setting of the stirring condition is accompanied by a limitation. Further, it is necessary to place and remove the work pieces into and out of the drum before and after the treatment and hence, during the placing and removal, a cracking and breaking may be caused in the work pieces.